This invention relates generally to magnetic flowmeter systems, and in particular to a magnetic flowmeter whose electromagnet is excited by DC power applied thereto periodically.
In a magnetic flowmeter, the liquid whose flow rate is to be measured, is conducted through a flow tube provided with a pair of diametrically-opposed electrodes and an electromagnet establishing a magnetic field perpendicular to the longitudinal axis of the tube. When the flowing liquid intersects the magnetic field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
Historically, in volumetric flow measurement utilizing a magnetic flowmeter, it was originally the practice to use a DC magnetic field. However, when employing a DC magnetic field, a polarization effect occurs. It is therefore now the practice to use an AC magnetic field in the magnetic flowmeter in order to overcome this drawback, for with an alternating magnetic flux operation, the polarization effect is rendered negligible.
Though an AC-excitation type of magnetic flowmeter is clearly advantageous in that polarization is obviated and the AC flow-induced signal may be more easily amplified, it has the following distinct drawbacks.
1. Eddy currents which flow through the fluid to be metered are induced by the use of AC magnetic field, and these eddy currents introduce error signals.
2. Because of stray capacitance between the loop from the AC excitation source to the electrodes and the fluid, the resultant electrostatic induction gives rise to zero drift.
3. Since the output signal of the magnetic flowmeter is alternating, the amplitude of the signal is reduced by reason of the electrostatic capacitance of the cable connection between the detector and a transmitter. This drawback is more pronounced in the case of flow measurement of low conductivity fluids.
4. A spurious voltage, which is 90 degrees out of phase with the AC flow induced voltage, is generated from the signal leads, thereby producing a changing zero voltage.